Multilevel power converter

ABSTRACT

Aspects of the invention are directed to a multilevel power converter the includes a DC power supply assembly having a positive terminal, a negative terminal, and a zero terminal; a first semiconductor switch series circuit composed of first through sixth semiconductor switches connected in series between the positive terminal and the negative terminal. The converter can include a second semiconductor switch series circuit composed of a first bidirectional switch, seventh and eighth semiconductor switches, and a second bidirectional switch connected in series between the connection point of the first and second semiconductor switches and the connection point of the fifth and sixth semiconductor switches, a first capacitor connected in parallel with a series circuit of the third and fourth semiconductor switches and a second capacitor connected in parallel with the second semiconductor switch series circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention are related to semiconductor powerconverter technology, and in particular, to technology related toconstructing multilevel power conversion circuits.

2. Description of the Related Art

FIG. 7 shows an example of seven level power conversion circuitaccording to a conventional technology disclosed in Japanese UnexaminedPatent Application Publication No. H11-164567. The circuit of FIG. 7 isa partial circuit for one phase of a multilevel, seven level in theconventional circuit of FIG. 7, power conversion circuit. A single phaseconversion circuit can be composed by using two partial circuits of FIG.7 and a three phase conversion circuit can be composed by using threepartial circuits of FIG. 7. Referring to FIG. 7, a DC power supplyassembly BA2 comprises series-connected single DC power supplies b11,b12, b13, b21, b22, and b23. The DC power supply assembly BA2 has seventerminals P1, P2, P3, M, N1, N2, and N3 at seven different voltagelevels. A series connection circuit of semiconductor switches Q1 throughQ12, which are IGBTs in this example, is connected between the positiveterminal P3 and the negative terminal N3. The connection point of thesemiconductors Q6 and Q7 is connected to an AC output terminal U. Adiode arm pair DA1 composed of series-connected diodes D1 and D2 isconnected between the connection point of the semiconductor switches Q1and Q2 and the connection point of the semiconductor switches Q7 and Q8.The middle terminal of the diode arm pair DA1 is connected to theconnection point of the single DC power supplies b11 and b12.

Similarly, a diode arm pair DA2 composed of series-connected diodes D3and D4 is connected between the connection point of the semiconductorswitches Q2 and Q3 and the connection point of the semiconductorswitches Q8 and Q9. The middle terminal of the diode arm pair DA2 isconnected to the connection point of the single DC power supplies b12and b13.

A diode arm pair DA3 composed of series-connected diodes D5 and D6 isconnected between the connection point of the semiconductor switches Q3and Q4 and the connection point of the semiconductor switches Q9 andQ10. The middle terminal of the diode arm pair DA3 is connected to theconnection point of the single DC power supplies b13 and b21.

The diode arm pair DA4 composed of series-connected diodes D7 and D8 isconnected between the connection point of the semiconductor switches Q4and Q5 and the connection point of the semiconductor switches Q10 andQ11. The middle terminal of the diode arm pair DA4 is connected to theconnection point of the single DC power supplies b21 and b22.

A diode arm pair DA5 composed of series-connected diodes D9 and D10 isconnected between the connection point of the semiconductor switches Q5and Q6 and the connection point of the semiconductor switches Q11 andQ12. The middle terminal of the diode arm pair DA5 is connected to theconnection point of the single DC power supplies b22 and b23.

In this circuit construction, when the semiconductor switches Q1 throughQ6 are in the ON state and the semiconductor switches Q7 through Q12 arein the OFF state, the AC terminal U outputs a voltage +3E; when thesemiconductor switches Q2 through Q7 are in the ON state and thesemiconductor switches Q8 through Q12 and Q1 are in the OFF state, theAC terminal U outputs a voltage +2E; when the semiconductor switches Q3through Q8 are in the ON state and the semiconductor switches Q9 throughQ12 and Q1 and Q2 are in the OFF state, the AC terminal U outputs avoltage +1E; when the semiconductor switches Q4 through Q9 are in the ONstate and the semiconductor switches Q10 through Q12 and Q1 through Q3are in the OFF state, the AC terminal U outputs a voltage 0 (zero); whenthe semiconductor switches Q5 through Q10 are in the ON state and thesemiconductor switches Q11 and Q12 and Q1 through Q4 are in the OFFstate, the AC terminal U outputs a voltage −1E; when the semiconductorswitches Q6 through Q11 are in the ON state and the semiconductorswitches Q12 and Q1 through Q5 are in the OFF state, the AC terminal Uoutputs a voltage −2E; and when the semiconductor switches Q7 throughQ12 are in the ON state and the semiconductor switches Q1 through Q6 arein the OFF state, the AC terminal U outputs a voltage −3E. Thus, the ACterminal U can deliver seven levels of output voltages by adjustingON/OFF of the semiconductor switches Q1 through Q12.

In the conventional circuit of FIG. 7, six series-connectedsemiconductor switches at the maximum carry the output current from theDC power supply assembly BA2 to the AC terminal U. This causes largesteady-state ON-state loss in the semiconductor switches anddeterioration of the overall conversion efficiency of the conversiondevice. Down-sizing and cost reduction are also difficult. Additionallyin the multilevel power conversion circuit as shown in FIG. 7, thesingle DC power supplies b11, b12, b13, b21, b22, and b23 do not bear anequal power in principle even though the output voltage and current fromthe AC terminal U have AC wave forms symmetrical with respect toelectrical polarity. Thus, the single DC power supplies need to beindependent on each other. The DC power supply assembly BA2, an inputdevice of the multilevel power conversion circuit, needs six single DCpower supplies each delivering a power independently, which imposes asevere limitation in production of the device. The problem of unbalanceof DC power supplies is disclosed in IEEE-PESC 1995 Conference Recordpp. 1144-1150 entitled “A multi-level voltage source converter systemwith balanced DC voltage.” Thus, as described above, there existscertain shortcomings in the art of DC power supplies.

SUMMARY OF THE INVENTION

Embodiments of the invention address these and other shortcomings.Embodiments of the invention provide a multilevel power converter thatgenerates reduced loss owing to reduced number of semiconductor switchescarrying output current as compared with conventional devices and thatcan be operated with a DC power supply assembly composed of only twosingle DC power supplies.

Some embodiments provide for a multilevel power converter for convertingDC power to AC power or AC power to DC power of a first aspect of theinvention that includes: a DC power supply assembly having first andsecond single DC power supplies connected in series, and three terminalsof a positive terminal, a negative terminal, and a middle point terminalat a middle electric potential between the positive terminal and thenegative terminal; a first semiconductor series circuit composed offirst through sixth semiconductor switches, each having anantiparallel-connected diode, connected in series in this order betweenthe positive terminal and the negative terminal of the DC power supplyassembly; a second semiconductor series circuit composed of a firstbidirectional switch, seventh and eighth semiconductor switches eachhaving an antiparallel-connected diode, and second bidirectional switchconnected in series in this order between a connection point of thefirst semiconductor switch and the second semiconductor switch and theconnection point of the fifth semiconductor switch and the sixthsemiconductor switch; a first capacitor connected in parallel with aseries circuit of the third semiconductor switch and the fourthsemiconductor switch; and a second capacitor connected in parallel withthe second semiconductor switch series circuit; wherein the zeroterminal is connected to a connection point of the seventh semiconductorswitch and the eighth semiconductor switch; and an AC terminal is at aconnection point of the third semiconductor switch and the fourthsemiconductor switch.

Some embodiments are directed to a second aspect of the invention,wherein the first semiconductor switch or the sixth semiconductor switchis composed of a plurality of semiconductor switches having the samefunction connected in series and the series-connected semiconductorswitches are each driven with a different control signal.

Some embodiments are directed to a multilevel power converter of a thirdaspect of the invention, wherein the first bidirectional switch or thesecond bidirectional switch is composed of semiconductor elements havingreverse blocking ability in antiparallel connection and theantiparallel-connected semiconductor elements are each driven with adifferent control signal.

Some embodiments are directed to a multilevel power converter device ofa fourth aspect of the invention, wherein the first bidirectional switchor the second bidirectional switch is composed of semiconductor switchesthat are series-connected with each other in an anti-series mode andeach of the semiconductor switches has an antiparallel connected diodeand driven with a different control signal.

Some embodiments are directed to a fifth aspect of the presentinvention, wherein the multilevel power converter uses the DC powersupply assembly having the three terminals at voltage levels of +3E, 0,and −3E, holds the first capacitor at a voltage of 1E and the secondcapacitor at a voltage of 2E, generates seven levels of voltages of +3E,+2E, +1E, 0, −1E, −2E, and −3E utilizing voltages of the DC power supplyassembly, the first capacitor, and the second capacitor, and deliversany voltage selected from the seven levels of voltages to the ACterminal.

Embodiments of the invention reduce the number of semiconductor switchescarrying the current from the input DC power supply assembly to the ACoutput terminal from six at the maximum to four at the maximum, thusdecreasing power loss. This can, in some circumstances, result in higherefficiency, lower cost, and smaller size. In addition, in someembodiments, the input DC power supply assembly can be composed of onlytwo single DC power supplies, which eliminates the limitation imposed onthe conventional circuit, making equipment production easier.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram showing a multilevel power converteraccording to a first embodiment of the invention;

FIG. 2 shows operation modes and operation waveform of the multilevelpower converter circuit according to the first embodiment of theinvention;

FIG. 3 shows the operation of semiconductor switches in the multilevelpower converter circuit according to the first embodiment of theinvention;

FIG. 4 is a circuit diagram showing a multilevel power converteraccording to a second embodiment of the invention;

FIG. 5 is a circuit diagram showing a multilevel power converteraccording to a third embodiment of the invention;

FIG. 6 shows the operation of semiconductor switches in the multilevelpower converter circuit according to the third embodiment of the presentinvention; and

FIG. 7 is a circuit diagram showing a multilevel power converter of aconventional technology.

DETAILED DESCRIPTION

A multilevel power converter of embodiments of the invention caninclude: a DC power supply assembly having a positive terminal, a zeroterminal, and a negative terminal; a first semiconductor switch seriescircuit composed of first through sixth semiconductor switches connectedin series in this order between the positive terminal and the negativeterminal; a second semiconductor switch series circuit composed of afirst bidirectional switch, seventh and eighth semiconductor switches,and a second bidirectional switch connected in series in this orderbetween the connection point of the first and second semiconductorswitches and the connection point of the fifth and sixth semiconductorswitches; a first capacitor connected in parallel with a series circuitof the third and fourth semiconductor switches; and a second capacitorconnected in parallel with the second semiconductor switch seriescircuit; wherein the zero terminal of the DC power supply assembly isconnected to the connection point of the seventh and eighthsemiconductor switches and an AC terminal is connected to the connectionpoint of the third and fourth semiconductor switches.

Embodiment 1

FIG. 1 shows a first embodiment of the invention. The circuit of FIG. 1is one phase of a power conversion circuit that uses a DC power supplycircuit of the DC power supply assembly BA1 having three terminals of apositive terminal, a zero terminal, and a negative terminal andcomprising series-connected single DC power supplies b1 and b2. Thismultilevel power conversion circuit comprises a first semiconductorswitch series circuit composed of series-connected semiconductorswitches Q1 through Q6 connected between the positive terminal P and thenegative terminal N of the DC power supply assembly BA1, and a firstcapacitor C1 connected in parallel with a series circuit ofsemiconductor switches Q3 and Q4. A second capacitor C2 is connectedbetween the connection point of semiconductor switches Q1 and Q2 and theconnection point of semiconductor switches Q5 and Q6. A secondsemiconductor switch series circuit is connected in parallel with thesecond capacitor C2. The second semiconductor switch series circuitcomprises a bidirectional switch SW1, the semiconductor switches Q7 andQ8, and another bidirectional switch SW2 connected in series. Theconnection point of the semiconductor switches Q7 and Q8 is connected tothe zero terminal M of the DC power supply assembly BA1, and theconnection point of the semiconductor switches Q3 and Q4 is connected tothe AC terminal U.

The following describes the operation of the circuit having theconstruction described above with reference to FIGS. 2 and 3. Thefollowing description is made for the seven level power conversioncircuit in which a voltage Vb1 of the single DC power supply b1 of 3E, avoltage Vb2 of the single DC power supply b2 of −3E, a voltage VC1 ofthe capacitor C1 of 1E and a voltage VC2 of the capacitor C2 of 2E.

There are 16 switching modes as shown in FIG. 2 and FIG. 3.

The switching mode 1 delivers +3E at the AC terminal U. In the switchingmode 1, the semiconductor switches Q1, Q2, and Q3 are in the ON stateand the semiconductor switch Q4 and the QR1 of the bidirectional switchSW1 are in the OFF state. The output current flows through the path: thesingle DC power supply b1→the semiconductor switch Q1→the semiconductorswitch Q2→the semiconductor switch Q3→the AC terminal U. The currentpasses through three semiconductor switches: Q1, Q2, and Q3. The voltageundergone by the semiconductor switch Q4 is equal to the voltage VC1across the capacitor C1, 1E, in the condition of OFF states of thesemiconductor switches Q5, Q6, and Q7 and the QR3 of the bidirectionalswitch SW2.

The semiconductor switch Q5 is subjected to a voltage VC2−VC1=1E that isthe voltage VC2 of the capacitor C2 (2E) subtracted by the voltage VC1of the capacitor C1 (1E). The semiconductor switch Q6 is subjected to avoltage equal to 4E that is the voltage−Vb2 of the single DC powersupply b2+the voltage Vb1 of the single DC power supply b1−the voltageVC2 of the capacitor C2=4E. The bidirectional switch SW2 is subjected toa voltage equal to 1E that is the voltage Vb1 of the single DC powersupply b1−the voltage VC2 of the capacitor C2=1E. The series-connectedcircuit of the semiconductor switch Q7 and the bidirectional switch SW1is subjected to a voltage of 3E that is the voltage Vb1 of the single DCpower supply b1. Here, if the semiconductor switch Q7 and thebidirectional switch SW1 of the series circuit thereof exhibit withstandvoltages with a ratio of two to one, the semiconductor switch Q7 issubjected to a voltage of 2E and the bidirectional switch SW1 issubjected to a voltage of 1E.

In the seven level power conversion circuit as described above, thevoltage at the AC terminal U is +3E under the conditions of: using theDC power supply assembly BA1 with the voltage Vb1 of the single DC powersupply b1 of 3E and the voltage Vb2 of the single DC power supply b2 of−3E, and holding the voltage VC1 of the capacitor C1 at 1E and thevoltage VC2 of the capacitor C2 at 2E. The bidirectional switches SW1and SW2 and semiconductor switches Q4, Q5, Q6, and Q7 are subjected tothe voltages 1E, 1E, 1E, 1E, 4E, and 2E, respectively.

The switching mode 2, with the assumption of seven level powerconversion circuit as in the switching mode 1 described above, delivers+2E at the AC terminal U. In this switching mode 2, the semiconductorswitches Q1, Q2, and Q4 are in the ON state and the semiconductorswitches Q3, Q5, Q6, and Q7, and QR1 of the bidirectional switch SW1 andQR3 of the bidirectional switch SW2 are in the OFF state. The outputcurrent flows through the path: the single DC power supply b1→thesemiconductor switch Q1→the semiconductor switch Q2→the capacitor C1→thesemiconductor switch Q4→the AC terminal U. The current passes throughthree semiconductor switches Q1, Q2, and Q4. The AC terminal U deliversa voltage of 2E that is the voltage Vb1 of the single DC power supply b1subtracted by the voltage VC1 of the capacitor C1. The semiconductorswitch Q3 is subjected to a voltage of 1E that is the voltage VC1; thesemiconductor switch Q5 is subjected to a voltage of 1E that is thevoltage VC2 of the capacitor C2 subtracted by the voltage VC1 of thecapacitor C1; the semiconductor switch Q6 is subjected to a voltage of4E that is the voltage−Vb2 of the single DC power supply b2+the voltageVb1 of the single DC power supply b1−the voltage VC2 of the capacitorC2; the bidirectional switch SW2 is subjected to a voltage of 1E that isthe voltage Vb1 of the single DC power supply b1 subtracted by thevoltage VC2 of the capacitor C2; and the series-connected circuit of thesemiconductor switch Q7 and the bidirectional switch SW1 is subjected tothe voltage Vb1 of the single DC power supply b1. These components areclamped at the voltages indicated above.

In the seven level power conversion circuit as described above, thevoltage at the AC terminal U is +2E under the conditions of: using theDC power supply assembly BA1 with the voltage Vb1 of the single DC powersupply b1 of 3E and the voltage Vb2 of the single DC power supply b2 of−3E, and holding the voltage VC1 of the capacitor C1 at 1E and thevoltage VC2 of the capacitor C2 at 2E. The bidirectional switches SW1and SW2 and the semiconductor switches Q3, Q5, Q6, and Q7 are subjectedto the voltages 1E, 1E, 1E, 1E, 4E, and 2E, respectively.

The switching mode 3, with the assumption of seven level powerconversion circuit as in the switching mode 1 described above, delivers+2E at the AC terminal U. In this switching mode 3, the semiconductorswitches Q1, Q3, and Q5 are in the ON state and the semiconductorswitches Q2, Q4, Q6, and Q7, and QR1 of the bidirectional switch SW1 andQR3 of the bidirectional switch SW2 are in the OFF state. The outputcurrent flows through the path: the single DC power supply b1→thesemiconductor switch Q1→the capacitor C2→the semiconductor switch Q5→thecapacitor C1→the semiconductor switch Q3→the AC terminal U. The currentpasses through three semiconductor switches Q1, Q5, and Q3. The ACterminal U delivers a voltage that is the voltage Vb1 of the single DCpower supply b1−the voltage VC2 of the capacitor C2+the voltage VC1 ofthe capacitor C1.

In the seven level power conversion circuit as described above, thevoltage at the AC terminal U is +2E under the conditions of: using theDC power supply assembly BA1 with the voltage Vb1 of the single DC powersupply b1 of 3E and the voltage Vb2 of the single DC power supply b2 of−3E, and holding the voltage VC1 of the capacitor C1 at 1E and thevoltage VC2 of the capacitor C2 at 2E. The bidirectional switches SW1and SW2 and the semiconductor switches Q2, Q4, Q6, and Q7 are clamped atthe voltages 1E, 1E, 1E, 1E, 4E, and 2E, respectively.

The switching mode 4, with the assumption of seven level powerconversion circuit as in the switching mode 1 described above, delivers+2E at the AC terminal U. In this switching mode 4, the bidirectionalswitches SW1 and SW2 and the semiconductor switches Q2, Q3, and Q8 arein the ON state, and the semiconductor switches Q1, Q4, Q5, Q6, and Q7are in the OFF state. The output current flows through the path: thezero terminal of the DC power supply assembly BA1→the semiconductorswitch Q8→the bidirectional switch SW2→the capacitor C2→thesemiconductor switch Q2→the semiconductor switch Q3→the AC terminal U.The current passes through four switches of the bidirectional switch SW2and the semiconductor switches Q8, Q2, and Q3. The AC terminal Udelivers directly a voltage of 2E that is the voltage VC2 of thecapacitor C2.

In the seven level power conversion circuit as described above, thevoltage at the AC terminal U is +2E under the conditions of: using theDC power supply assembly BA1 with the voltage Vb1 of the single DC powersupply b1 of 3E and the voltage Vb2 of the single DC power supply b2 of−3E, and holding the voltage VC1 of the capacitor C1 at 1E and thevoltage VC2 of the capacitor C2 at 2E. The semiconductor switches Q1,Q4, Q5, Q6, and Q7 are clamped at the voltages 1E, 1E, 1E, 3E, and 2E,respectively.

The switching mode 5, with the assumption of seven level powerconversion circuit as described above, delivers +1E at the AC terminalU. In this switching mode 5, the semiconductor switches Q1, Q4, and Q5are in the ON state, and the semiconductor switches Q2, Q3, Q6, and Q7and the QR1 of the bidirectional switch SW1, and the QR3 of thebidirectional switch SW2 are in the OFF state. The output current flowsthrough the path: the single DC power supply b1→the semiconductor switchQ1→the capacitor C2→the semiconductor switch Q5→the semiconductor switchQ4→the AC terminal U. The current passes through three switches of thesemiconductor switches Q1, Q5, and Q4. The AC terminal U delivers avoltage of 1E that is the voltage Vb1 of the single DC power supplyb1−the voltage VC2 of the capacitor C2.

In the seven level power conversion circuit as described above, thevoltage at the AC terminal U is +1E under the conditions of: using theDC power supply assembly BA1 with the voltage Vb1 of the single DC powersupply b1 of 3E and the voltage Vb2 of the single DC power supply b2 of−3E, and holding the voltage VC1 of the capacitor C1 at 1E and thevoltage VC2 of the capacitor C2 at 2E. The bidirectional switches SW1and SW2, the semiconductor switches Q2, Q3, Q6, and Q7 are clamped atthe voltages 1E, 1E, 1E, 1E, 4E, and 2E, respectively.

The switching mode 6, with the assumption of seven level powerconversion circuit as described above, delivers +1E at the AC terminalU. In this switching mode 6, the bidirectional switches SW1 and SW2, thesemiconductor switches Q2, Q4, and Q8 are in the ON state, and thesemiconductor switches Q1, Q3, Q5, Q6, and Q7 are in the OFF state. Theoutput current flows through the path: the zero terminal M of the DCpower supply assembly BA1→the semiconductor switch Q8→the bidirectionalswitch SW2→the capacitor C2→the semiconductor switch Q2→the capacitorC1→the semiconductor switch Q4→the AC terminal U. The current passesthrough four switches of the semiconductor switches Q8, Q2, and Q4 andthe bidirectional switch SW2. The AC terminal U delivers a voltage of 1Ethat is the voltage VC2 of the capacitor C2−the voltage VC1 of thecapacitor C1.

In the seven level power conversion circuit as described above, thevoltage at the AC terminal U is +1E under the conditions of: using theDC power supply assembly BA1 with the voltage Vb1 of the single DC powersupply b1 of 3E and the voltage Vb2 of the single DC power supply b2 of−3E, and holding the voltage VC1 of the capacitor C1 at 1E and thevoltage VC2 of the capacitor C2 at 2E. The semiconductor switches Q1,Q3, Q5, Q6, and Q7 are clamped at the voltages 1E, 1E, 1E, 3E, and 2E,respectively.

The switching mode 7, with the assumption of seven level powerconversion circuit as described above, delivers +1E at the AC terminalU. In this switching mode 7, the bidirectional switches SW1 and SW2, thesemiconductor switches Q3, Q5, and Q8 are in the ON state, and thesemiconductor switches Q1, Q2, Q4, Q6, and Q7 are in the OFF state. Theoutput current flows through the path: the zero terminal M of the DCpower supply assembly BA1→the semiconductor switch Q8→the bidirectionalswitch SW2→the semiconductor switch Q5→the capacitor C1→semiconductorswitch Q3→the AC terminal U. The current passes through four switches ofthe semiconductor switches Q8, Q5, and Q3 and the bidirectional switchSW2. The AC terminal U delivers a voltage of 1Ed that is the voltage VC1of the capacitor C1.

In the seven level power conversion circuit as described above, thevoltage at the AC terminal U is +1E under the conditions of: using theDC power supply assembly BA1 with the voltage Vb1 of the single DC powersupply b1 of 3E and the voltage Vb2 of the single DC power supply b2 of−3E, and holding the voltage VC1 of the capacitor C1 at 1E and thevoltage VC2 of the capacitor C2 at 2E. The semiconductor switches Q1,Q2, Q4, Q6, and Q7 are clamped at the voltages 1E, 1E, 1E, 3E, and 2E,respectively.

The switching mode 8, with the assumption of seven level powerconversion circuit as described above, delivers 0 (zero) at the ACterminal U. In this switching mode 8, the bidirectional switches SW1 andSW2, the semiconductor switches Q4, Q5, and Q8 are in the ON state, andthe semiconductor switches Q1, Q2, Q3, Q6, and Q7 are in the OFF state.The output current flows through the path: the zero terminal M of the DCpower supply assembly BA1→the semiconductor switch Q8→the bidirectionalswitch SW2→the semiconductor switch Q5→the semiconductor switch Q4→theAC terminal U. The current passes through four switches of thesemiconductor switches Q8, Q5, and Q4 and the bidirectional switch SW2.The AC terminal U delivers a voltage of 0 (zero) that is the potentialat the zero terminal M of the DC power supply assembly BA1.

In the seven level power conversion circuit as described above, thevoltage at the AC terminal U is 0 (zero) under the conditions of: usingthe DC power supply assembly BA1 with the voltage Vb1 of the single DCpower supply b1 of 3E and the voltage Vb2 of the single DC power supplyb2 of −3E, and holding the voltage VC1 of the capacitor C1 at 1E and thevoltage VC2 of the capacitor C2 at 2E. The semiconductor switches Q1,Q2, Q3, Q6, and Q7 are clamped at the voltages 1E, 1E, 1E, 3E, and 2E,respectively.

The switching mode 9, with the assumption of seven level powerconversion circuit as described above, delivers 0 (zero) at the ACterminal U. In this switching mode 9, the bidirectional switches SW1 andSW2, the semiconductor switches Q2, Q3, and Q7 are in the ON state, andthe semiconductor switches Q1, Q4, Q5, Q6, and Q8 are in the OFF state.The output current flows through the path: the zero terminal M of the DCpower supply assembly BA1→the semiconductor switch Q7→the bidirectionalswitch SW1→the semiconductor switch Q2→the semiconductor switch Q3→theAC terminal U. The current passes through four switches of thesemiconductor switches Q7, Q2, and Q3 and the bidirectional switch SW1.The AC terminal U delivers a voltage of 0 (zero) that is the potentialat the zero terminal M of the DC power supply assembly BA1.

In the seven level power conversion circuit as described above, thevoltage at the AC terminal U is 0 under the conditions of: using the DCpower supply assembly BA1 with the voltage Vb1 of the single DC powersupply b1 of 3E and the voltage Vb2 of the single DC power supply b2 of−3E, and holding the voltage VC1 of the capacitor C1 at 1E and thevoltage VC2 of the capacitor C2 at 2E. The semiconductor switches Q1,Q4, Q5, Q6, and Q8 are clamped at the voltages 3E, 1E, 1E, 1E, and 2E,respectively.

The switching mode 10, with the assumption of seven level powerconversion circuit as described above, delivers −1E at the AC terminalU. In this switching mode 10, the bidirectional switches SW1 and SW2,the semiconductor switches Q2, Q4, and Q7 are in the ON state, and thesemiconductor switches Q1, Q3, Q5, Q6, and Q8 are in the OFF state. Theoutput current flows through the path: the zero terminal M of the DCpower supply assembly BA1→the semiconductor switch Q7→the bidirectionalswitch SW1→the semiconductor switch Q2→the capacitor C1→thesemiconductor switch Q4→the AC terminal U. The current passes throughfour switches of the semiconductor switches Q7, Q2, and Q4 and thebidirectional switch SW1. The AC terminal U delivers a voltage of −1Ethat is the voltage−VC1 of the capacitor C1.

In the seven level power conversion circuit as described above, thevoltage at the AC terminal U is −1E under the conditions of: using theDC power supply assembly BA1 with the voltage Vb1 of the single DC powersupply b1 of 3E and the voltage Vb2 of the single DC power supply b2 of−3E, and holding the voltage VC1 of the capacitor C1 at 1E and thevoltage VC2 of the capacitor C2 at 2E. The semiconductor switches Q1,Q3, Q5, Q6, and Q8 are clamped at the voltages 3E, 1E, 1E, 1E, and 2E,respectively.

The switching mode 11, with the assumption of seven level powerconversion circuit as described above, delivers −1E at the AC terminalU. In this switching mode 11, the bidirectional switches SW1 and SW2,the semiconductor switches Q3, Q5, and Q7 are in the ON state, and thesemiconductor switches Q1, Q2, Q4, Q6, and Q8 are in the OFF state. Theoutput current flows through the path: the zero terminal M of the DCpower supply assembly BA1→the semiconductor switch Q7→the bidirectionalswitch SW1→the capacitor C2→the semiconductor switch Q5→the capacitorC1→the semiconductor switch Q3→the AC terminal U. The current passesthrough four switches of the semiconductor switches Q7, Q5, and Q3 andthe bidirectional switch SW1. The AC terminal U delivers a voltage of−1E that is the voltage−VC2 of the capacitor C2+the voltage VC1 of thecapacitor C1.

In the seven level power conversion circuit as described above, thevoltage at the AC terminal U is −1E under the conditions of: using theDC power supply assembly BA1 with the voltage Vb1 of the single DC powersupply b1 of 3E and the voltage Vb2 of the single DC power supply b2 of−3E, and holding the voltage VC1 of the capacitor C1 at 1E and thevoltage VC2 of the capacitor C2 at 2E. The semiconductor switches Q1,Q2, Q4, Q6, and Q8 are clamped at the voltages 3E, 1E, 1E, 1E, and 2E,respectively.

The switching mode 12, with the assumption of seven level powerconversion circuit as described above, delivers −1E at the AC terminalU. In this switching mode 12, the semiconductor switches Q2, Q3, and Q6are in the ON state, and the semiconductor switches Q1, Q4, Q5, and Q8,and the QR1 of the bidirectional switch SW1 and the QR3 of thebidirectional switch SW2 are in the OFF state. The output current flowsthrough the path: the single DC power supply b2→the semiconductor switchQ6→the capacitor C2→the semiconductor switch Q2→the semiconductor switchQ3→the AC terminal U. The current passes through three switches of thesemiconductor switches Q6, Q2, and Q3. The AC terminal U delivers avoltage of −1E that is the voltage Vb2 of the single DC power supplyb2+VC2 of the capacitor C2.

In the seven level power conversion circuit as described above, thevoltage at the AC terminal U is −1E under the conditions of: using theDC power supply assembly BA1 with the voltage Vb1 of the single DC powersupply b1 of 3E and the voltage Vb2 of the single DC power supply b2 of−3E, and holding the voltage VC1 of the capacitor C1 at 1E and thevoltage VC2 of the capacitor C2 at 2E. The bidirectional switches SW1and SW2, semiconductor switches Q1, Q4, Q5, and Q8 are clamped at thevoltages 1E, 1E, 4E, 1E, 1E, and 2E, respectively.

The switching mode 13, with the assumption of seven level powerconversion circuit as described above, delivers −2E at the AC terminalU. In this switching mode 13, the bidirectional switches SW1 and SW2,and the semiconductor switches Q4, Q5, and Q7 are in the ON state, andthe semiconductor switches Q1, Q2, Q3, Q6, and Q8, are in the OFF state.The output current flows through the path: the zero terminal M of the DCpower supply assembly BA1→the semiconductor switch Q7→the bidirectionalswitch SW1→the capacitor C2→the semiconductor switch Q5→thesemiconductor switch Q4→the AC terminal U. The current passes throughfour switches of the semiconductor switches Q7, Q5, and Q4 and thebidirectional switch SW1. The AC terminal U delivers a voltage of −2Ethat is the voltage−VC2 of the capacitor C2.

In the seven level power conversion circuit as described above, thevoltage at the AC terminal U is −2E under the conditions of: using theDC power supply assembly BA1 with the voltage Vb1 of the single DC powersupply b1 of 3E and the voltage Vb2 of the single DC power supply b2 of−3E, and holding the voltage VC1 of the capacitor C1 at 1E and thevoltage VC2 of the capacitor C2 at 2E. The semiconductor switches Q1,Q2, Q3, Q6, and Q8 are clamped at the voltages 3E, 1E, 1E, 1E, and 2E,respectively.

The switching mode 14, with the assumption of seven level powerconversion circuit as described above, delivers −2E at the AC terminalU. In this switching mode 14, the semiconductor switches Q2, Q4, and Q6are in the ON state, and the semiconductor switches Q1, Q3, Q5, and Q8,and the QR2 of the bidirectional switch SW1 and the QR4 of thebidirectional switch SW2 are in the OFF state. The output current flowsthrough the path: the single DC power supply b2→the semiconductor switchQ6→the capacitor C2→the semiconductor switch Q2→the capacitor C1→thesemiconductor switch Q4→the AC terminal U. The current passes throughthree switches of the semiconductor switches Q6, Q2, and Q4. The ACterminal U delivers a voltage of −2E that is the voltage Vb2 of thesingle DC power supply b2+the voltage VC2 of the capacitor C2−thevoltage VC1 of the capacitor C1.

In the seven level power conversion circuit as described above, thevoltage at the AC terminal U is −2E under the conditions of: using theDC power supply assembly BA1 with the voltage Vb1 of the single DC powersupply b1 of 3E and the voltage Vb2 of the single DC power supply b2 of−3E, and holding the voltage VC1 of the capacitor C1 at 1E and thevoltage VC2 of the capacitor C2 at 2E. The bidirectional switches SW1and SW2 and the semiconductor switches Q1, Q3, Q5, and Q8 are clamped atthe voltages 1E, 1E, 4E, 1E, 1E, and 2E, respectively.

The switching mode 15, with the assumption of seven level powerconversion circuit as described above, delivers −2E at the AC terminalU. In this switching mode 15, the semiconductor switches Q3, Q5, and Q6are in the ON state, and the semiconductor switches Q1, Q2, Q4, and Q8,and the QR2 of the bidirectional switch SW1 and the QR4 of thebidirectional switch SW2 are in the OFF state. The output current flowsthrough the path: the single DC power supply b2→the semiconductor switchQ6→the semiconductor switch Q5→the capacitor C1→the semiconductor switchQ3→the AC terminal U. The current passes through three switches of thesemiconductor switches Q6, Q5, and Q3. The AC terminal U delivers avoltage of −2E that is the voltage Vb2 of the single DC power supplyb2+the voltage VC1 of the capacitor C1.

In the seven level power conversion circuit as described above, thevoltage at the AC terminal U is −2E under the conditions of: using theDC power supply assembly BA1 with the voltage Vb1 of the single DC powersupply b1 of 3E and the voltage Vb2 of the single DC power supply b2 of−3E, and holding the voltage VC1 of the capacitor C1 at 1E and thevoltage VC2 of the capacitor C2 at 2E. The bidirectional switches SW1and SW2 and the semiconductor switches Q1, Q2, Q4, and Q8 are clamped atthe voltages 1E, 1E, 4E, 1E, 1E, and 2E, respectively.

The switching mode 16, with the assumption of seven level powerconversion circuit as described above, delivers −3E at the AC terminalU. In this switching mode 16, the semiconductor switches Q4, Q5, and Q6are in the ON state, and the semiconductor switches Q1, Q2, Q3, and Q8,and the QR2 of the bidirectional switch SW1 and the QR4 of thebidirectional switch SW2 are in the OFF state. The output current flowsthrough the path: the single DC power supply b2→the semiconductor switchQ6→the semiconductor switch Q5→the semiconductor switch Q4→the ACterminal U. The current passes through three switches of thesemiconductor switches Q6, Q5, and Q4. The AC terminal U delivers avoltage of −3E that is the voltage Vb2 of the single DC power supply b2.

In the seven level power conversion circuit as described above, thevoltage at the AC terminal U is −3E under the conditions of: using theDC power supply assembly BA1 with the voltage Vb1 of the single DC powersupply b1 of 3E and the voltage Vb2 of the single DC power supply b2 of−3E, and holding the voltage VC1 of the capacitor C1 at 1E and thevoltage VC2 of the capacitor C2 at 2E. The bidirectional switches SW1and SW2 and the semiconductor switches Q1, Q2, Q3, and Q8 are clamped atthe voltages 1E, 1E, 4E, 1E, 1E, and 2E, respectively.

The seven level power conversion circuit with the single DC powersupplies b1 and b2 delivering voltages Vb1 of +3E and Vb2 of −3E,respectively, can output seven levels of voltages of +3E, +2E, +1E, 0,−1E, −2E, and −3E at the AC terminal U by the switching mode 1 throughthe switching mode 16.

The seven level power conversion circuit operates to deliver the sameoutput voltage of +2E in the switching mode 2 through the switching mode4. However, in the positive direction of the AC output current i fromthe AC terminal, the switching mode 2 charges the capacitor C1 with theAC output current i; the switching mode 3 discharges the capacitor C1and charges the capacitor C2; and the switching mode 4 discharges thecapacitor C2.

The same voltage of +1E is delivered from the AC terminal U in theswitching mode 5 through switching mode 7. The switching mode 5 chargesthe capacitor C2; the switching mode 6 charges the capacitor C1 anddischarges the capacitor C2; and the switching mode 7 discharges thecapacitor C1. Thus, the voltage VC1 of the capacitor C1 and the voltageVC2 of the capacitor C2 can be adjusted independently by appropriatelyselecting, for delivering a voltage of +2E at the AC output terminal U,a switching mode from the switching modes 2, 3, and 4, and selecting,for delivering a voltage of +1E at the AC terminal U, a switching modefrom the switching modes 5, 6, and 7. The example of seven level powerconversion circuit described above controls the voltage VC1 of thecapacitor C1 at 1E and the voltage VC2 of the capacitor C2 at 2E.

Because of the symmetry of the circuit, the similar relationship holdsin the switching mode 10 through the switching mode 15. In the switchingmode 1, switching mode 8, switching mode 9, and switching mode 16, nocurrent flows through the capacitor C1 and the capacitor C2. Thus, thevoltage VC1 of the capacitor C1 and the voltage VC2 of the capacitor C2do not change.

Embodiment 2

FIG. 4 shows a multilevel power converter circuit according to a secondembodiment of the invention. The device of this Embodiment 2 differsfrom the device of Embodiment 1 in the construction of bidirectionalswitches SW1 and SW2. While bidirectional switches in the circuit of theEmbodiment 1 are constructed by antiparallel connection of reverseblocking semiconductor switches QR1 through QR4, bidirectional switchesin Embodiment 2 are constructed by anti-series connection ofsemiconductor switches QS1 through QS4 without reverse blocking ability.In operation of the bidirectional switches of the Embodiment 2, theswitching of the QS1 is conducted in the same ON/OFF operation as of theQR1; QS2, in the same ON/OFF operation as of the QR2; QS3, in the sameON/OFF operation as of the QR3; and QS4, in the same ON/OFF operation asof the QR4. Thus, the bidirectional switches SW1 and SW2 perform exactlythe same operation and function in both Embodiment 1 and Embodiment 2.

Embodiment 3

FIG. 5 shows a multilevel power converter circuit of a third embodimentaccording to the invention. The device of this Embodiment 3 differs fromthe device of Embodiment 1 in that the semiconductor switch Q1 inEmbodiment 1 is replaced by a series-connected circuit of semiconductorswitches Q1 a and Q1 b in Embodiment 3 and the semiconductor switch Q6in Embodiment 1 is replaced by a series-connected circuit ofsemiconductor switches Q6 a and Q6 b in Embodiment 3.

FIG. 6 shows a seven level operation in the device with the replacedsemiconductor switches Q1 a, Q1 b, Q6 a, and Q6 b. In the switchingmodes 1, 2, 3, and the switching mode 5, the switches Q1 a and Q1 b areboth in the ON state and the switches Q6 a and Q6 b are both in the OFFstate. In the switching mode 4 and the switching modes 6, 7, and 8, oneof the switches Q1 a and Q1 b is in the ON state and both the Q6 a andQ6 b are in the OFF state. In the switching modes 9, 10, 11, and theswitching mode 13, the Q1 a and Q1 b are both in the OFF state and oneof the Q6 a and Q6 b is in the ON state. In the switching mode 12 andthe switching modes 14, 15, and 16, the Q1 a and Q1 b are both in theOFF state and the Q6 a and Q6 b are both in the ON state.

In this operation shown in FIG. 6 of the device of FIG. 5, thesemiconductor switches Q1 and Q6 in FIG. 1, each of which needs awithstand voltage of at least 4E in principle, can be replaced by theseries-connected circuit of the semiconductor switches Q1 a and Q1 b andby the series-connected circuit of the semiconductor switches Q6 a andQ6 b, each of the Q1 a, Q1 b, Q6 a, and Q6 b being necessary to haveonly half the withstand voltage of the Q1 and Q6. Transition between themodes in this device is carried out by single switching operation of asemiconductor element of a low withstand voltage, which reducesswitching loss and thus achieves lower loss in the device. In IGBTs ofhigh withstand voltage of in the range of kilovolts or higher, forexample, switching characteristics and steady-state loss deterioraterapidly with increase in the withstand voltage. This technology of theinvention is useful in such a case to reduce power loss employingsemiconductor elements of low withstand voltage, which exhibit excellentcharacteristics

The devices of the embodiments described thus far are inverters forconverting a DC power to an AC power. However, the present invention canbe applied to a converter circuit for converting an AC power to a DCpower.

The present invention provides a circuit technology for generating amultilevel AC voltage from a small number of single DC power supplies,and can be applied to high voltage motor driving equipments and powerconverters for grid-connection.

Examples of specific embodiments are illustrated in the accompanyingdrawings. While the invention is described in conjunction with thesespecific embodiments, it will be understood that it is not intended tolimit the invention to the described embodiments. On the contrary, it isintended to cover alternatives, modifications, and equivalents as may beincluded within the spirit and scope of the invention as defined by theappended claims. In the above description, specific details are setforth in order to provide a thorough understanding of embodiments of theinvention. Embodiments of the invention may be practiced without some orall of these specific details. Further, portions of differentembodiments and/or drawings can be combined, as would be understood byone of skill in the art.

This application is based on, and claims priority to, Japanese PatentApplication No. 2012-091629, filed on Apr. 13, 2012, contents of whichare incorporated herein by reference.

What is claimed is:
 1. A multilevel power converter comprising: a DCpower supply assembly having first and second single DC power suppliesconnected in series, and three terminals of a positive terminal, a zeroterminal, and a negative terminal; a first semiconductor switch seriescircuit composed of first through sixth semiconductor switches, eachhaving an antiparallel-connected diode, connected in series in thisorder between the positive terminal and the negative terminal of the DCpower supply assembly; a second semiconductor switch series circuitcomposed of a first bidirectional switch, seventh and eighthsemiconductor switches each having an antiparallel-connected diode, andsecond bidirectional switch connected in series in this order between aconnection point of the first semiconductor switch and the secondsemiconductor switch and the connection point of the fifth semiconductorswitch and the sixth semiconductor switch; a first capacitor connectedin parallel with a series circuit of the third semiconductor switch andthe fourth semiconductor switch; and a second capacitor connected inparallel with the second semiconductor switch series circuit; whereinthe zero terminal of the DC power supply assembly is connected to aconnection point of the seventh semiconductor switch and the eighthsemiconductor switch; and an AC terminal is at a connection point of thethird semiconductor switch and the fourth semiconductor switch.
 2. Themultilevel power converter according to claim 1, wherein the firstsemiconductor switch or the sixth semiconductor switch is composed of aplurality of semiconductor switches having the same function connectedin series and the series-connected semiconductor switches are eachdriven with a different control signal.
 3. The multilevel powerconverter according to claim 1, wherein the first bidirectional switchor the second bidirectional switch is composed of semiconductor elementshaving reverse blocking ability in antiparallel connection and theantiparallel-connected semiconductor elements are each driven with adifferent control signal.
 4. The multilevel power converter according toclaim 2, wherein the first bidirectional switch or the secondbidirectional switch is composed of semiconductor elements havingreverse blocking ability in antiparallel connection and theantiparallel-connected semiconductor elements are each driven with adifferent control signal.
 5. The multilevel power converter according toclaim 1, wherein the first bidirectional switch or the secondbidirectional switch is composed of semiconductor switches that areseries-connected with each other in an anti-series mode and each of thesemiconductor switches has an antiparallel connected diode and is drivenwith a different control signal.
 6. The multilevel power converteraccording to claim 2, wherein the first bidirectional switch or thesecond bidirectional switch is composed of semiconductor switches thatare series-connected with each other in an anti-series mode and each ofthe semiconductor switches has an antiparallel connected diode and isdriven with a different control signal.
 7. The multilevel powerconverter according to claim 1, wherein the multilevel power converteruses the DC power supply assembly having the three terminals at voltagelevels of +3E, 0, and −3E, holds the first capacitor at a voltage of 1Eand the second capacitor at a voltage of 2E, generates seven levels ofvoltages of +3E, +2E, +1E, 0, −1E, −2E, and −3E utilizing voltages ofthe DC power supply assembly, the first capacitor, and the secondcapacitor, and delivers any voltage selected from the seven levels ofvoltages to the AC terminal.
 8. The multilevel power converter accordingto claim 2, wherein the multilevel power converter uses the DC powersupply assembly having the three terminals at voltage levels of +3E, 0,and −3E, holds the first capacitor at a voltage of 1E and the secondcapacitor at a voltage of 2E, generates seven levels of voltages of +3E,+2E, +1E, 0, −1E, −2E, and −3E utilizing voltages of the DC power supplyassembly, the first capacitor, and the second capacitor, and deliversany voltage selected from the seven levels of voltages to the ACterminal.